The invention relates to a micro device used in inertial force sensor, optical switch or the like, and particularly to a micro device comprising an insulating substrate and a beam-like structure made of silicon formed on the insulating substrate, and a method of manufacturing the same.
Recently it has been made possible to etch silicon as deep as 100 xcexcm by means of reactive ion etching (RIE) technology using inductively coupled plasma (ICP) as the activation energy source (hereinafter referred to as ICP-RIE process). This technique is viewed as a promising new technique for making silicon structures of high aspect ratios with a sufficiently high etching rate, in the field of device development by a micromachining. In the past, the wet process using an alkali solution was predominant as the process of deep etching of silicon substrates. But it is difficult to make a desired structure by the wet process, because the direction of etching depends on the crystal orientation of silicon in the wet process. In contrast, the ICP-RIE process is not subject to anisotropy of etching because it is a dry process. Thus the ICP-RIE process has such an advantage over the wet process that far higher degree of freedom in designing the configuration of structure can be achieved than in the case of the wet process.
When machining by dry etching a silicon substrate whereon a mask film has been formed in a desired pattern by photolithography or the like, however, there occurs such a problem that a wider area (exposed through a wider aperture) is etched at a higher rate than a narrower area. This is caused by micro loading effect, which is a well-known phenomenon in the field of semiconductor manufacturing processes. This phenomenon has such an adverse effect as described below on the micro devices which fall in the scope of the present invention, namely micro devices comprising an insulating substrate and a beam-like structure made of silicon formed on the insulating substrate.
FIG. 15 and FIG. 16 show the structure of an inertial force sensor as an example of basic structure of a micro device 100 of the prior art. FIG. 15 is a schematic plan view and FIG. 16 is a sectional view taken along lines XVI-XVIxe2x80x2 of FIG. 15. The inertial force sensor 100 comprises an insulating substrate 101 having a recess formed in the surface thereof, and a beam-like structure 104 made of silicon so as to interpose the recess on the surface of the insulating substrate 101. The beam-like structure 104 further comprises two electrodes 105, 105. The electrode 105 comprises a supporting section 106 and a plurality of cantilevers 107. The cantilevers 107 are arranged to oppose each other via a minute clearance.
FIGS. 17A-17G are sectional views schematically showing the manufacturing process of the inertial force sensor shown in FIG. 15 of the prior art. A similar manufacturing process has been proposed, for example, by Z. Xiao et al. in Proc. of Transducers xe2x80x299, pp. 1518-1521, and S. Kobayashi et al. in Proc. of Transducers xe2x80x299, pp. 910-913.
A silicon substrate 103 is provided in the step of FIG. 17A, and a glass substrate 101 is provided in the step of FIG. 17B. A mask film 108 is formed on the surface of the glass substrate 101 by photolithography in the step of FIG. 17C, and a recess 102 is formed by etching the surface of the glass substrate 101 to a depth in a range from several micrometers to several tens of micrometers with a dilute solution of hydrofluoric acid in the step of FIG. 17D. In the step of FIG. 17E, the silicon substrate 103 is bonded onto the surface of the glass substrate 101 by anodic bonding. In the step of FIG. 17F, a mask film 109 having a pattern that corresponds to the planar configuration of the beam-like structure 104 shown in FIG. 15 is formed by photolithography. In the step of FIG. 17G, the silicon substrate 103 is etched through by the ICP-RIE process, to form a cantilever 107. Then the resist remaining on the surface of the silicon substrate is removed.
The step of FIG. 17G involves a problem. The mask film 109 in the step of FIG. 17F generally has both of wide apertures and narrow apertures. Consequently, when a dry etching process such as the ICP-RIE process is applied to the silicon substrate 103 that has the mask film 109, the silicon substrate is etched at a higher rate in a portion exposed through the wider aperture than in a portion exposed through the narrower aperture due to the micro loading effect. As a result, the wider portion is etched through earlier than the narrower portion in the silicon substrate 103. At this time, etching gas enters into the clearance between the recess 102 of the glass substrate 101 and the back surface of the silicon substrate 103 through the hole which has been etched out in the silicon substrate 103 earlier. The etching gas which has entered erodes the back surface of the silicon substrate 103 till the narrowest portion is completely etched out. Thus the side wall of the supporting section 106 and the bottom surface or the side wall of the cantilever 107 are eroded. As a result, dimensions of the beam-like structure 104 deviate significantly from the designed values, making it impossible to obtain the target characteristics of the device.
Erosion of the supporting section and the cantilever due to the micro loading effect can be restricted by making the sizes of all apertures comparable when designing. However, this approach imposes severe limitation to the freedom of designing the device structure. Even when the dimensions of apertures are set to be the same in design, it is difficult to completely prevent the erosion of the supporting section and the cantilever in the actual process. This is because it is a common practice to apply over-etching to some extent in order to etch through reliably.
An object of the present invention is therefore to provide a micro device which has a beam-like structure that provides a sufficient degree of freedom in the design of the device structure by restricting the erosion of the supporting section and the cantilever due to the micro loading effect, and a method of manufacturing the same.
The present inventors have completed the present invention by finding that the problem described above can be solved by a micro device having an electrically conductive film which is formed on a recessed surface at least in a portion right under a cantilever of an insulating substrate and is electrically connected with a supporting section.
Specifically, the micro device of the present invention comprises an insulating substrate having a recess formed on the surface thereof, and a beam-like structure made of silicon formed on the front surface of the insulating substrate to surround the recess, wherein the beam-like structure comprises at least one functional section and the functional section has a supporting section bonded onto the insulating substrate and at least one cantilever formed integrally with the supporting section while extending across the recess. The micro device also has an electrically conductive film formed on the surface of the recess at least in a portion right under a cantilever.
The micro device of the present invention has the following features.
Erosion of the supporting section made of silicon and the cantilever is caused, as described above, by the etching gas which enters into the clearance between the recess of the insulating substrate and the back surface of the silicon substrate which has been etched through earlier during the dry etching process. The silicon substrate is etched in such a mechanism of dry etching as activated ions having positive charge are accelerated by a negative bias formed right above the silicon substrate thereby to collide with the silicon substrate with a sufficient energy. In the case of the ICP-RIE process, the activated etching gas is usually sulfur fluoride ion (SFx+). The ion turns into silicon fluoride (SiFx) through reaction with silicon, and is discharged to the outside. The negative bias is formed above the silicon substrate by applying a high frequency to a substrate holder that also serves as a cathode whereon the silicon substrate is placed.
Therefore, erosion of the back surface of the silicon substrate is considered to occur as the SFx+ that has entered the clearance between the back surface of the silicon substrate and the recess of the insulating substrate is repulsed by the surface of the insulating substrate and collides with the back surface of the silicon substrate. Repulsion of the SFx+ on the surface of the insulating substrate may be caused also by electrical repulsion force as well as kinematic scattering. The electrical repulsion force will be described below with reference to FIG. 13 and FIG. 14.
FIG. 13 is a schematic sectional view showing a silicon substrate 45 bonded onto the surface of an insulating substrate 41, which has a recess, so as to surround the recess 42, in a state of the silicon substrate 45 being dry-etched. The silicon substrate 45 has a mask film 50 formed on the surface thereof for the purpose of forming a functional section. The silicon substrate 45 is formed into a supporting section 46 and a plurality of cantilevers 47 through dry etching.
During the dry etching process, the surface of the recess 42 of the insulating substrate 41 is charged with positive charge 52 by the etching gas which impinges thereon a number of times, for example, SFx+51. The surface of the recess 42 charged with the positive charge repulses the SFx+51 which comes next. The repulsed SFx+51 changes the direction of the movement thereof before reaching the recess 42 and instead impinges on the back surface of the silicon substrate 45. Also it may be that the SFx+51 which is bound to hit the insulating substrate 41 at right angles is distracted from the trajectory by the recess 42 that is positively charged, and impinges on the side wall of the supporting section 46.
Therefore, in order to restrict the erosion of the back surface of the silicon substrate 45 or the supporting section 46, it is effective to prevent the surface of the recess 42 of the insulating substrate 41 from being positively charged.
According to the present invention, as shown in FIG. 14, the electrically conductive film 43 is formed on the surface of the recess 42 of the insulating substrate 41, and the surface of the recess 42 of the insulating substrate 41 is prevented from being positively charged by electrically connecting the electrically conductive film 43 and the supporting section 46. In this case, when the etching gas collides with the electrically conductive film 43, charge of the etching gas is discharged through the supporting section 46, thereby deactivating the etching gas. Since the silicon substrate 45 has the same potential as the substrate holder which is held at a negative potential during dry etching, charge of the etching gas is neutralized upon collision with the electrically conductive film 43, so that deactivation is accelerated.
While it suffices to form the electrically conductive film used in the present invention on the surface of the recess at least in a portion right under the cantilever, it is preferable to use the electrically conductive film formed over the entire surface of the recessed, which makes it possible to prevent the entire surface of the recess from being charged thereby restricting the erosion of the back surface of the silicon substrate more effectively.
The inertial force sensor of the present invention comprises an insulating substrate having a recess formed on the surface thereof, and a beam-like structure made of silicon formed on the front surface of the insulating substrate so as to interpose the recess, wherein the beam-like structure comprises a movable electrode and a fixed electrode, with the movable electrode and the fixed electrode each having a supporting section bonded onto the insulating substrate and a comb-shaped electrode comprising a plurality of cantilever electrodes formed integrally with the supporting section while extending across the recess. The cantilevers of the movable electrode and the fixed electrode are arranged to oppose each other via a minute clearance. In the inertial force sensor having such a constitution, an electrically conductive film which is electrically connected with the supporting section is formed on the surface of the recess at least in a portion right under the cantilever.
The inertial force sensor of the present invention has the electrically conductive film which is formed on the surface of the recess at least in a portion right under the cantilever, for the purpose of preventing the surface from being charged, and is electrically connected with the supporting section. Thus when the cantilever is formed by dry etching, the supporting section and the cantilever are not subject to erosion because the etching gas having positive charge loses the charge upon collision with the electrically conductive film and is neutralized. As a result, since there occurs no variation in the distance between the cantilevers that constitute the movable electrode and the comb-shaped electrode of the fixed electrode, such an inertial force sensor can be provided as the deterioration of sensitivity and variation in the characteristic are suppressed.
The micro device of the present invention comprises an insulating substrate having a recess formed on the surface thereof, a beam-like structure made of silicon formed on the front surface of the insulating substrate so as to interpose the recess, an optical fiber holder which is fastened on the beam-like structure and holds a plurality of optical fibers disposed at a predetermined distance, and electromagnetic attraction means fastened to oppose the back surface of the insulating substrate and the beam-like structure. The beam-like structure comprises a supporting section which has an aperture and cantilevers formed integrally with the supporting section. The supporting section is bonded onto the insulating substrate and has a fixed mirror provided at one end of the inner wall of the aperture, while the cantilever is formed to overhang from the other end of the inner wall of the aperture, with a movable mirror being provided to erect on the surface at the tip of the cantilever to oppose the fixed mirror. A magnetic film that reacts with the electromagnetic attraction means is formed on the back surface of the cantilever, so that the electromagnetic attraction means attracts the back surface of the tip of the cantilever onto the recess of the insulating substrate, via the magnetic film, thereby switching the mirror, that reflects the light incident from the optical fiber, from the movable mirror to the fixed mirror, thus switching the optical path and allowing the application as an optical switch.
The optical switch described above has an electrically conductive film which is formed on the surface of a recessed in a portion at least right under the cantilever of the insulating substrate and is electrically connected with the supporting section. As a result, when the silicon substrate is processed to form the cantilever by reactive etching, the etching gas having positive charge collides with the electrically conductive film and loses the charge thereby to be deactivated, and therefore the etching gas does not erode the back surface of the cantilever. Thus since the cantilever having a high accuracy of the profile is formed, such an optical switch can be provided as the deterioration of response characteristic during switching of the optical path and variation in the characteristics are suppressed.
The method of manufacturing the micro device of the present invention, which comprises the insulating substrate having the recess formed on the surface thereof and the beam-like structure made of silicon formed on the front surface of the insulating substrate so as to interpose the recess, wherein the beam-like structure comprises at least one functional section and the functional section has the supporting section bonded onto the insulating substrate and at least one cantilever formed integrally with the supporting section while extending across the recess, comprises a step of forming the electrically conductive film on the surface of the recess at least in a portion right under the cantilever of the insulating substrate and extending the electrically conductive film over the surface around the recess thereby to establish electrical continuity with a supporting section; a step of forming a first mask film which corresponds to the configuration of the supporting section on the surface of the silicon substrate; a step of forming the supporting section by etching the surface of the silicon substrate whereon the first mask film has been formed; a step of bonding the silicon substrate which has the supporting section and the insulating substrate which has the electrically conductive film so that the surfaces thereof oppose each other; a step of forming a second mask film which corresponds to the configuration of the cantilever on the back surface of the silicon substrate that has been bonded; and a step of etching the back surface of the silicon substrate having the second mask film formed thereon to penetrate through the silicon substrate by dry etching, thereby to form the cantilever of a desired pattern which extends across the recess.
According to the manufacturing method of the present invention, the electrically conductive film is formed on the surface of the recess of the insulating substrate for the purpose of preventing the surface from being charged. At this time, a part of the electrically conductive film is extended over the surface around the recess thereby to form an electrical lead to the supporting section. As the electrically conductive film is electrically connected to the supporting section, the electrically conductive film is kept at the same potential as the substrate holder which is electrically connected to the supporting section, and is subjected to a negative bias. Thus when etching the back surface of the silicon substrate, which has the second mask film formed thereon, thereby to penetrate through the silicon substrate by dry etching, the etching gas having positive charge collides with the electrically conductive film and loses the charge thereby to be deactivated, and therefore the etching gas does not erode the back surface of the cantilever. As a result, since the side wall of the supporting section and the bottom surface or side wall of the cantilever are not eroded, it is not necessary to design the apertures of the mask film to have similar dimensions. Thus the present invention can provide the manufacturing method of the micro device having the high accuracy beam-like structure made of silicon and a high degree of freedom of design.
For the dry etching process to form the cantilever, it is desirable to employ the ICP-RIE process, which makes it possible to form the beam-like structure mad of silicon having a high aspect ratio in a shorter period of time.